Recently I have been started getting error like below

Unable to cast object of type 'Microsoft.Practices.SharePoint.Common.Configuration.FarmSettingStoreForUpgrade' to type 'Microsoft.Practices.SharePoint.Common.Configuration.FarmSettingStore'.Time :8/27/2015 2:09:34 PM

   at Microsoft.SharePoint.Administration.SPPersistedObject.GetChild[T](String name)
   at Microsoft.Practices.SharePoint.Common.Configuration.FarmSettingStore.Load(SPFarm farm)
   at Microsoft.Practices.SharePoint.Common.Configuration.SPFarmPropertyBag.GetSettingStore()
   at Microsoft.Practices.SharePoint.Common.Configuration.SPFarmPropertyBag.get_Item(String key)
   at Microsoft.Practices.SharePoint.Common.Configuration.ConfigManager.GetProperty(Type settingType, String key, IPropertyBag propertyBag)
   at Microsoft.Practices.SharePoint.Common.Configuration.ConfigManager.GetFromPropertyBag[TValue](String key, IPropertyBag propertyBag)
   at Microsoft.Practices.SharePoint.Common.ServiceLocation.ServiceLocatorConfig.GetConfigData()
   at Microsoft.Practices.SharePoint.Common.ServiceLocation.ServiceLocatorConfig.GetTypeMappings()
   at Microsoft.Practices.SharePoint.Common.ServiceLocation.SharePointServiceLocator.CreateServiceLocatorInstance()
   at Microsoft.Practices.SharePoint.Common.ServiceLocation.SharePointServiceLocator.GetCurrentFarmLocator()
   at Microsoft.Practices.SharePoint.Common.ServiceLocation.SharePointServiceLocator.DoGetCurrent()
   at Microsoft.Practices.SharePoint.Common.ServiceLocation.SharePointServiceLocator.GetCurrent()

I am using

Microsoft.Practices.ServiceLocation.dll &
Microsoft.Practices.SharePoint.Common.dll 

since last 6 months for my applications for configuration store and logging functionality. Never faced any issue untl now.

Uninstalled both the DLLs from the GAC and reinstalled, reset the IIS and restarted the Timer Jobs, but still the error persists.

Hi,

Please try to add one dll into GAC, open a new PowerShell window then deploy the second.

Here is a thread for your reference:

https://social.msdn.microsoft.com/Forums/office/en-US/3be7efbe-bc55-486f-9b64-458c9257edb4/microsoftpracticessharepointcommondll-the-only-way-this-works-is-after-a-server-reboot?forum=sharepointadminprevious

Or try to restart your server to check whether it works.

Best Regards,

Dennis